This invention relates to the stabilization of salts derived from a lactam and an alkali metal.
Lactams of the general formula ##EQU1## are employed as starting materials for the preparation of high molecular weight polyamides. The homopolymer of .epsilon.-caprolactam is often referred to as "nylon 6". These lactam polymers exhibit excellent tensile properties and can be formed into a variety of useful products including textile denier fibers, films, castings and molded objects. Pure, dry lactams, particularly .epsilon.-caprolactam, do not polymerize appreciably even at elevated temperatures; however the polymerization can be activated using any one of a large number of materials as catalysts. Alkali metal salt of lactams are particularly effective catalysts, and are discussed by W. E. Hanford and R. M. Joyce [Journal of Polymer Science, Volume 3, No. 2, pages 167-172 (1948)]. The polymerization proceeds rapidly at temperatures between 200.degree. and 280.degree.C. and is conveniently carried out by adding the desired amount of alkali metal to the lactam to effect, first, salt formation, followed by a polymerization which is catalyzed by the salt. A major shortcoming of the alkali metal salts of lactams is that they discolor relatively rapidly even under inert atmospheres, particularly at the elevated temperatures employed to liquefy and polymerize lactams. The color imparted to the resultant polymer renders it unacceptable for many end uses, particularly as textile fiber and film.
It is therefore an object of the present invention to eliminate or at least significantly reduce the discoloration which occurs in compositions containing an alkali metal salt of a lactam.
A second objective of this invention is to stabilize polymerizable mixtures containing a lactam and an alkali metal salt of said lactam without adversely affecting the activity of the alkali metal salt as a latent polymerization catalyst.
It has now been found that these objectives can be attained when a hexaorgano distannane is present in combination with an alkali metal lactam salt.